Modern communications networks are increasingly based on silica optical fiber which offers very wide bandwidth within several spectral wavelength bands. At the transmitter end of a typical point-to-point fiber optic communications link, an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1525-1565 nanometer transmission band (the so-called C-band), and the resulting modulated optical signal is coupled into one end of the silica optical fiber. On sufficiently long links, the optical signal may be directly amplified along the route by one or more amplifiers, for example, optically-pumped erbium-doped fiber amplifiers (EDFAs). At the receiving end of the fiber link, a photodetector receives the modulated light and converts it back to its original electrical form. For very long links, the optical signal risks becoming excessively distorted due to fiber-related impairments, such as, chromatic and polarization dispersion, and by noise limitations of the amplifiers, and may be reconstituted by detecting and re-launching the signal back into the fiber. This process is typically referred to as optical-electrical-optical (OEO) regeneration.
In recent developments, the transmission capacity of fiber optic systems has been greatly increased by wavelength division multiplexing (WDM) in which multiple independent optical signals, differing uniquely by wavelength, are simultaneously transmitted over the fiber optic link. For example, the C-band transmission window has a bandwidth of about 35 nanometers, determined partly by the spectral amplification bandwidth of an EDFA amplifier, in which multiple wavelengths may be simultaneously transmitted. All else being equal, for a WDM network containing N number of wavelengths, the data transmission capacity of the link is increased by a factor of N. Depending on the specifics of a WDM network, the wavelength multiplexing into a common fiber is typically accomplished with devices employing a diffraction grating, an arrayed waveguide grating, or a series of thin-film filters. At the receiver of a WDM system, the multiple wavelengths can be spatially separated using the same types of devices that performed the multiplexing, and, then separately detected and output in their original electrical data streams.
Dense WDM (DWDM) systems are being designed in which the transmission spectrum includes 40, 80, or more wavelengths with wavelength spacing of less than 1 nanometer. Current designs have wavelength spacing of between 0.4 and 0.8 nanometer, or equivalently a frequency spacing of 50 to 100 GHz respectively. Spectral packing schemes allow for higher or lower spacing, dictated by economics, bandwidth, and other factors. Other amplifier types, for example Raman, that help to expand the available WDM spectrum are currently being commercialized. However, the same issues about signal degradation and OEO regeneration exist for WDM as with non-WDM fiber links. The expense of OEO regeneration is compounded by the large number of wavelengths present in WDM systems.
Modern fiber optic networks are evolving to be much more complicated than the simple point-to-point “long haul” systems described above. Instead, as fiber optic networks move into the regional, metro, and local arenas, they increasingly include multiple nodes along the fiber span, and connections between fiber spans (e.g., mesh networks and interconnected ring networks) at which signals received on one incoming link can be selectively switched between a variety of outgoing links, or taken off the network completely for local consumption. For electronic links, or optical signals that have been detected and converted to their original electrical form, conventional electronic switches directly route the signals to their intended destination, which may then include converting the signals to the optical domain for fiber optic transmission. However, the desire to switch fiber optic signals while still in their optical format, thereby avoiding expensive OEO regeneration to the largest extent possible, presents a new challenge to the switching problem.
Switching
In the most straightforward and traditional fiber switching approach, each network node that interconnects multiple fiber links includes a multitude of optical receivers, which convert the signals from optical to electrical form, a conventional electronic switch which switches the electrical data signals, and an optical transmitter which converts the switched signals from electrical back to optical form. In a WDM system, this optical/electrical/optical (OEO) conversion must be performed by separate receivers and transmitters for each of the W wavelength components on each fiber. This replication of expensive OEO components is currently slowing the implementation of highly interconnected mesh WDM systems employing a large number of wavelengths.
Another approach for fiber optic switching, implements sophisticated wavelength switching in an all-optical network. In one version of this approach, the wavelength components W from an incoming multi-wavelength fiber are de-multiplexed into different spatial paths. Individual and dedicated switching elements then route the wavelength-separated signals toward the desired output fiber port before a multiplexer aggregates the optical signals of differing wavelengths onto a single outgoing fiber. In conventional fiber switching systems, all the fiber optic switching elements and associated multiplexers and de-multiplexers are incorporated into a wavelength selective switch (WSS), which is a special case of an enhanced optical cross connect (OXC) having a dispersive element and wavelength-selective capability. Additionally, such systems incorporate lenses and mirrors which focus and reflect light, and lenslets which collimate such light.
Advantageously, all the fiber optic switching elements can be implemented in a single chip of a micro electromechanical system (MEMS). The MEMS chip generally includes a two-dimensional array of tiltable mirrors which may be separately controlled. U.S. Pat. No. 6,097,859 to Solgaard et al., describes the functional configuration of such a MEMS wavelength selective switch (WSS), which accepts wavelengths from an incoming fiber and is capable of switching them to any one of multiple outgoing fibers. The entire switching array of up to several hundred micro electromechanical system (MEMS) mirrors, can be fabricated on a chip having dimensions of less than one centimeter by techniques well developed in the semiconductor integrated circuit industry.
Solgaard et al. further describe a large multi-port (including multiple input M and multiple output N fiber ports) and multi-wavelength WDM wavelength selective switch (WSS), accomplishing this by splitting the WDM channels into their wavelength components W and switching those wavelength components W. The WSS of Solgaard et al. has the capability of switching any wavelength channel on any input fiber port to the corresponding wavelength channel on any output fiber port. Again, a wavelength channel on any of the input fibers can be switched to the same wavelength channel on any of the output fibers. Each MEMS mirror in today's WDM wavelength selective switch is dedicated to a single wavelength channel whether it tilts about one or more axes.
As fiber port counts increase, however, the size of the optics of such WDM wavelength selective switches grows quickly. In turn, the size of the device increases, and the switching element(s) must provide a greater spatial path deflection of the wavelength components. For example, where a MEMS mirror array is employed, the increased size of the device requires a greater tilt angle, increasing the cost of the MEMS mirror array, and increasing the defect rate. Furthermore, many such WDM wavelength selective switches require elements dedicated to a particular special path, i.e., tuned for a particular fiber port. Such dedicated elements increase costs by virtue of their number, but also typically require extremely high performance characteristics and low tolerances, which, likewise, increases costs.
Therefore, it is readily apparent that there is a need for an improved WDM wavelength selective switch that allows for increased fiber port counts without substantially increasing the size of the device, and at the same time, reduces the performance requirements for the components thereof, including the switching elements.